Contents

Cisco Response

This Applied Mitigation Bulletin is a companion document to the PSIRT Security Advisory Multiple Vulnerabilities in Cisco Firewall Services Module and provides identification and mitigation techniques that administrators can deploy on Cisco network devices.

Vulnerability Characteristics

There are multiple vulnerabilities in Cisco Firewall Services Module. The following subsections summarize these vulnerabilities:

Sun Remote Procedure Call (SunRPC) Inspection Denial of Service Vulnerabilities: These vulnerabilities can be exploited remotely without authentication and without end-user interaction. Successful exploitation of these vulnerabilities may cause the affected device to crash and reload, resulting in a denial of service (DoS) condition. Repeated attempts to exploit these vulnerabilities could result in a sustained DoS condition. The attack vector for exploitation is through SunRPC packets using UDP port 111. An attacker could exploit these vulnerabilities using spoofed packets.

These vulnerabilities have been assigned CVE identifiers CVE-2010-2818, CVE-2010-2819, and CVE-2010-2820.

TCP Denial of Service Vulnerability: This vulnerability can be exploited remotely without authentication and without end-user interaction. Successful exploitation of this vulnerability may cause the affected device to crash, resulting in a denial of service (DoS) condition. Repeated attempts to exploit this vulnerability could result in a sustained DoS condition.

The attack vectors for exploitation are through packets using the following protocols and ports:

Mitigation Technique Overview

Cisco devices provide several countermeasures for these vulnerabilities. Administrators are advised to consider these protection methods to be general security best practices for infrastructure devices and the traffic that transits the network. This section of the document provides an overview of these techniques.

Cisco IOS® Software can provide effective means of exploit prevention using Infrastructure access control lists (iACLs). This protection mechanism filters and drops packets that are attempting to exploit these vulnerabilities.

Effective exploit prevention can also be provided by the Cisco ASA 5500 Series Adaptive Security Appliance and the Firewall Services Module (FWSM) for Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers using Transit access control lists (tACLs). This protection mechanism filters and drops packets that are attempting to exploit these vulnerabilities.

Effective use of Cisco Intrusion Prevention System (IPS) event actions provides visibility into and protection against attacks that attempt to exploit these vulnerabilities.

Cisco IOS Software, Cisco ASA, and FWSM firewalls can provide visibility through syslog messages and counter values displayed in the output from show commands.

The Cisco Security Monitoring, Analysis, and Response System (Cisco Security MARS) appliance can also provide visibility through incidents, queries, and event reporting.

Risk Management

Organizations are advised to follow their standard risk evaluation and mitigation processes to determine the potential impact of these vulnerabilities. Triage refers to sorting projects and prioritizing efforts that are most likely to be successful. Cisco has provided documents that can help organizations develop a risk-based triage capability for their information security teams. Risk Triage for Security Vulnerability Announcements and Risk Triage and Prototyping can help organizations develop repeatable security evaluation and response processes.

Device-Specific Mitigation and Identification

Caution: The effectiveness of any mitigation technique depends on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. As with any configuration change, evaluate the impact of this configuration prior to applying the change.

Specific information about mitigation and identification is available for these devices:

Mitigation: Infrastructure Access Control Lists

To protect infrastructure devices and minimize the risk, impact, and effectiveness of direct infrastructure attacks, administrators are advised to deploy infrastructure access control lists (iACLs) to perform policy enforcement of traffic sent to infrastructure equipment. Administrators can construct an iACL by explicitly permitting only authorized traffic sent to infrastructure devices in accordance with existing security policies and configurations. For the maximum protection of infrastructure devices, deployed iACLs should be applied in the ingress direction on all interfaces to which an IP address has been configured. An iACL workaround cannot provide complete protection against these vulnerabilities when the attack originates from a trusted source address.

The iACL policy denies unauthorized packets that are sent to affected devices via the following protocols:

SSH using TCP port 22

Telnet using TCP port 23

Cisco Adaptive Security Device Manager (ASDM) SSL using TCP port 443

In the following example, 192.168.60.0/24 is the IP address space that is used by the affected devices, and the host at 192.168.100.1 is considered a trusted source that requires access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic. Whenever possible, infrastructure address space should be distinct from the address space used for user and services segments. Using this addressing methodology will assist with the construction and deployment of iACLs.

Note that filtering with an interface access list will elicit the transmission of ICMP unreachable messages back to the source of the filtered traffic. Generating these messages could have the undesired effect of increasing CPU utilization on the device. In Cisco IOS Software, ICMP unreachable generation is limited to one packet every 500 milliseconds by default. ICMP unreachable message generation can be disabled using the interface configuration command no ip unreachables. ICMP unreachable rate limiting can be changed from the default using the global configuration command ip icmp rate-limit unreachableinterval-in-ms.

Identification: Infrastructure Access Control Lists

After the administrator applies the iACL to an interface, the show ip access-lists command will identify the number of SSH, Telnet, and SSL packets on TCP ports 22, 23, and 443 that have been filtered on interfaces on which the iACL is applied. Administrators should investigate filtered packets to determine whether they are attempts to exploit these vulnerabilities. Example output for show ip access-lists Infrastructure-ACL-Policy follows:

Administrators can use Embedded Event Manager to provide instrumentation when specific conditions are met, such as ACE counter hits. The Applied Intelligence white paper Embedded Event Manager in a Security Context provides additional details about how to use this feature.

Identification: Access List Logging

The log and log-input access control list (ACL) option will cause packets that match specific ACEs to be logged. The log-input option enables logging of the ingress interface in addition to the packet source and destination IP addresses and ports.

Caution: Access control list logging can be very CPU intensive and must be used with extreme caution. Factors that drive the CPU impact of ACL logging are log generation, log transmission, and process switching to forward packets that match log-enabled ACEs.

The CPU impact from ACL logging can be addressed in hardware on the Cisco Catalyst 6500 Series switches and Cisco 7600 Series routers with Supervisor Engine 720 or Supervisor Engine 32 using optimized ACL logging.

Identification: Traffic Flow Identification Using NetFlow Records

Administrators can configure Cisco IOS NetFlow on Cisco IOS routers and switches to aid in the identification of traffic flows that may be attempts to exploit these vulnerabilities. Administrators are advised to investigate flows to determine whether they are attempts to exploit these vulnerabilities or whether they are legitimate traffic flows.

Mitigation: Transit Access Control Lists

To protect the network from traffic that enters the network at ingress access points, which may include Internet connection points, partner and supplier connection points, or VPN connection points, administrators are advised to deploy tACLs to perform policy enforcement. Administrators can construct a tACL by explicitly permitting only authorized traffic to enter the network at ingress access points or permitting authorized traffic to transit the network in accordance with existing security policies and configurations. A tACL workaround cannot provide complete protection against these vulnerabilities when the attack originates from a trusted source address.

The tACL policy denies unauthorized packets that are sent to affected devices via the following protocols:

In the following example, 192.168.60.0/24 is the IP address space that is used by the affected devices, and the host at 192.168.100.1 is considered a trusted source that requires access to the affected devices. Care should be taken to allow required traffic for routing and administrative access prior to denying all unauthorized traffic.

Identification: Transit Access Control Lists

After the tACL has been applied to an interface, administrators can use the show access-list command to identify the number of SSH, Telnet, and SSL packets on TCP ports 22, 23, and 443 that have been filtered. Administrators are advised to investigate filtered packets to determine whether they are attempts to exploit these vulnerabilities. Example output for show access-list tACL-Policy follows:

In the following example, the show logging | grepregex command extracts syslog messages from the logging buffer on the firewall. These messages provide additional information about denied packets that could indicate potential attempts to exploit the vulnerabilities that are described in this document. It is possible to use different regular expressions with the grep keyword to search for specific data in the logged messages.

In the preceding example, the messages logged for the tACL tACL-Policy show SSH packets for TCP port 22, Telnet packets for TCP port 23, and SSL packets for TCP port 443 sent to the address block assigned to the affected devices.

Mitigation: Cisco IPS Signature Event Actions

Administrators can use Cisco Intrusion Prevention System (IPS) appliances and services modules to provide threat detection and help prevent attempts to exploit the vulnerabilities that are described in this document. These vulnerabilities may be detected by the following signatures:

24120/0 - Cisco ASA RPC Vulnerability

24140/0 - Cisco ASA RPC Vulnerability

24159/0 - Cisco ASA RPC Vulnerability

24120/0 - Cisco ASA RPC Vulnerability

Beginning with signature update S505 for sensors running Cisco IPS version 6.x and greater, this vulnerability can be detected by signature 24120/0 (Signature Name: Cisco ASA RCP Vulnerability). Signature 24120/0 is enabled by default, triggers a High severity event, has a signature fidelity rating (SFR) of 90, and is configured with a default event action of produce-alert.

Signature 24120/0 fires when a specific attempt at exploiting the vulnerability as documented by Cisco identifier CSCtc85753 is detected. Firing of this signature may indicate a potential exploit of this vulnerability.

24140/0 - Cisco ASA RPC Vulnerability

Beginning with signature update S505 for sensors running Cisco IPS version 6.x and greater, this vulnerability can be detected by signature 24140/0 (Signature Name: Cisco ASA RPC Vulnerability). Signature 24140/0 is enabled by default, triggers a High severity event, has a signature fidelity rating (SFR) of 90, and is configured with a default event action of produce-alert.

Signature 24140/0 fires when a specific attempt at exploiting the vulnerability documented by Cisco identifier CSCte61662 is detected. Firing of this signature may indicate a potential exploit of this vulnerability.

24159/0 - Cisco ASA RPC Vulnerability

Beginning with signature update S505 for sensors running Cisco IPS version 6.x and greater, this vulnerability can be detected by signature 24159/0 (Signature Name: Cisco ASA RPC Vulnerability). Signature 24159/0 is enabled by default, triggers a High severity event, has a signature fidelity rating (SFR) of 90, and is configured with a default event action of produce-alert.

Signature 24159/0 fires when a specific attempt at exploiting the vulnerability documented by Cisco identifier CSCte61710 is detected. Firing of this signature may indicate a potential exploit of this vulnerability.

Administrators can configure Cisco IPS sensors to perform an event action when an attack is detected. The configured event action performs preventive or deterrent controls to help protect against an attack that is attempting to exploit the vulnerabilities that are described in this document.

Exploits that use spoofed IP addresses may cause a configured event action to inadvertently deny traffic from trusted sources.

Cisco IPS sensors are most effective when deployed in inline protection mode combined with the use of an event action. Automatic Threat Prevention for Cisco IPS 6.x and greater sensors that are deployed in inline protection mode provides threat prevention against an attack that is attempting to exploit the vulnerabilities that are described in this document. Threat prevention is achieved through a default override that performs an event action for triggered signatures with a riskRatingValue greater than 90.

The Cisco Security Monitoring, Analysis, and Response System (Cisco Security MARS) appliance can create incidents regarding events that are related to the vulnerabilities that are described in this document using IPS signatures 24120/0 (Signature Name: Cisco ASA RPC Vulnerability), 24140/0 (Signature Name: Cisco ASA RPC Vulnerability) and 24159/0 (Signature Name: Cisco ASA RPC Vulnerability). After the S505 dynamic signature update has been downloaded, using keyword NR-24120/0 for IPS signature 24120/0, NR-24140/0 for IPS signature 24140/0, or NR-24159/0 for IPS signature 24159/0 and a query type of All Matching Event Raw Messages on the Cisco Security MARS appliance will provide a report that lists the incidents created by the IPS signature.

Beginning with the 4.3.1 and 5.3.1 releases of Cisco Security MARS appliances, support for the Cisco IPS dynamic signature updates feature has been added. This feature downloads new signatures from Cisco.com or from a local web server, correctly processes and categorizes received events that match those signatures, and includes them in inspection rules and reports. These updates provide event normalization and event group mapping, and they also enable the MARS appliance to parse new signatures from the IPS devices.

Caution: If dynamic signature updates are not configured, events that match these new signatures appear as unknown event type in queries and reports. Because MARS will not include these events in inspection rules, incidents may not be created for potential threats or attacks that occur within the network.

By default, this feature is enabled but requires configuration. If it is not configured, the following Cisco Security MARS rule will be triggered:

System Rule: CS-MARS IPS Signature Update Failure

When this feature is enabled and configured, administrators can determine the current signature version downloaded by MARS by selecting Help > About and reviewing the IPS Signature Version value.

Additional information about dynamic signature updates and instructions for configuring dynamic signature updates are available for the Cisco Security MARS 4.3.1 and 5.3.1 releases.

Additional Information

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